script installation problem - addon not showing up in UI

I switched to 2.78 recently. I added (user-preferences - addons) the 3D Print toolbox addon. When I start Blender, then I see the addon.
If I open an older Blender file (pre 2.78) with 2.78, then the add-on is not showing up. Even if I copy the script to the addons folder. Even if I try to re-install it in this file through user-preferences. The addon is checkmarked/installed there. But its not shown in the UI/panel.

??How can I solve that??

I have seen an error message that the script was in 2 directories. I removed it from one directory. Now there is no error message anymore. But the addon is still not in the panel (although marked as installed in the preferences).

All add-ons are there when I start an empty Blender 2.78
When I open an older .blend from 2.78, then the 3D print toolbox is missing, even when I explicitely de-activate
-Load User Interface Setup in the .blend-file- while doing so.
And the 3D print toolbox addon is activated in the addons panel.

Wow, how can I make this work?

Also, it seems to be only this one file I have trouble getting the proper UI/panel with the addons. When I open other, older .blend files in 2.78, then I have the proper 2.78 UI with all addons shown…
And the system console does not show any errors either.

How can I get the proper UI/panels/addons in this file?

Can’t you just append the contents of that very file to a new one?
Other than that: How are we supposed to troubleshoot issues with a special file without access to that said file?